2016 Outlander Sport with 5-speed manual transmission, 76K miles on odometer. Intermittent high revs (up to 4K, usually more like 3K) when OUT of gear, coasting on the street or highway. Placing the gear shift into 2nd or 3rd, with clutch depressed, will bring revs down again. However the cycle may repeat in a short time. Overall MPG good (33 or so) on trips, on regular gas. Power improved with new plugs, but this is never going to be a race car! Additionally, car may come close to stalling at a light, revs drop to around 500 until it comes back up to 1K by itself. I think the two rev symptoms are related. Has been checked for codes, none found. Mid grade gas had no discernible effect on power or these symptoms.
Vacuum leak perhaps? How to check?
These symptoms most likely would not be noticeable on a car equipped with an automatic, since it stays in gear 100% of the time while driving!
Appreciate anyone's insights on this!